Limit state design of steel beams, columns, and connections per IS 800:2007 — India's general construction in steel code. Covers section classification, flexure with LTB, shear, column compression, and partial safety factors.
IS 800:2007 adopted the Limit State Method (LSM) in its third revision, replacing the Working Stress Method. The code covers hot-rolled, cold-formed (IS 801 for cold-formed separately), and built-up sections.
| Failure Mode | γm | Clause |
|---|---|---|
| Yielding (gross section) | γm0 = 1.10 | IS 800 Cl.5.4.1 |
| Rupture (net section) | γm1 = 1.25 | IS 800 Cl.5.4.1 |
| Bolt bearing / friction | γmb = 1.25 | IS 800 Cl.5.4.1 |
| Welds | γmw = 1.25 | IS 800 Cl.5.4.1 |
IS 800 Cl.3.7 classifies sections into four classes based on the width-to-thickness ratios of the compression elements, similar to Eurocode 3:
| Class | Description | b/t limit (flange, IS 800) |
|---|---|---|
| Class 1 (Plastic) | Full plastic moment; allows moment redistribution | ≤ 8.4ε |
| Class 2 (Compact) | Full plastic moment; no redistribution | ≤ 9.4ε |
| Class 3 (Semi-compact) | Elastic moment only (yield at extreme fibre) | ≤ 13.6ε |
| Class 4 (Slender) | Local buckling before yield; effective section required | > 13.6ε |
Where ε = √(250/fy). For E250: ε = 1.0. For E350: ε = √(250/350) = 0.845.
For laterally restrained beams (LTB not possible), Md = Zp·fy/γm0. Lateral restraint requires the compression flange to be braced at adequate intervals.
When the compression flange is not braced, LTB reduces the moment capacity. IS 800 uses a buckling curve approach similar to Eurocode 3:
The elastic critical moment Mcr depends on the unbraced length LLT, section properties (Iy, Iw, J), and loading pattern (uniform moment, midpoint load, etc.).
For high shear (V > 0.6·Vd), the moment capacity is reduced. Web shear buckling must be checked for d/tw > 67ε.
| Section Type | Axis | Buckling Curve | α |
|---|---|---|---|
| Hot-rolled I-section (h/b > 1.2, tf ≤ 40 mm) | Major (y-y) | a | 0.21 |
| Hot-rolled I-section (h/b > 1.2, tf ≤ 40 mm) | Minor (z-z) | b | 0.34 |
| Hot-rolled I-section (h/b ≤ 1.2) | Both | b | 0.34 |
| Welded I-section | Major | b | 0.34 |
| Welded I-section | Minor | c | 0.49 |
| Hollow sections (hot-finished) | Both | a | 0.21 |
| Cold-formed hollow sections | Both | c | 0.49 |
Cm is the equivalent uniform moment factor (IS 800 Table 18). PE is the Euler buckling load for the member. The check is performed for both major and minor axes independently.